In electron accelerators using acceleration voltages up to 150 kV it is known to use high vacuum for insulation. Accelerators of this kind have been used successfully for many years now in industry.
On the other hand, in electron accelerators constructed for acceleration voltages over 150 kV, the charged particle source (electrons or ions), and the post-acceleration stages, are still installed in a tank filled with an insulating gas at a pressure higher than atmospheric.
Charged-particle accelerators of this kind have the following disadvantages, in particular:
A. Whenever it becomes necessary to attend to the source of charged particles (replacement of the cathode in an electron accelerator, renewal of the gas or other material in an ion accelerator), the insulating gas has to be pumped out of the tank, stored, cleaned and finally pumped back into the tank, a time-consuming and costly operation. This applies particularly to the very costly SF.sub.6, which is a good electron-negative gas but is very sensitive to moisture.
B. Due to presence of the pressure tank the accelerator has to be officially inspected at frequent intervals.
C. The pressure tank is of costly construction.
D. The post-accelerating tube contains a high vacuum and needs to be sealed carefully against the pressure tank. The seals used are usually rubber rings in contact with glass or ceramic. The individual sections of the apparatus must of course also be electrically insulated from each other. The constructional parts of the post-accelerating tube therefore have to perform two different functions. They have to act as gas seals and at the same time as voltage insulators.
E. The known accelerators are of complex and costly construction and are unsuitable in this regard for general use on a large scale in industry, where robustness and simple serviceing are essential.